You should realize that 128 has a very different meaning in adobeRGB and prophotoRGB than it has in sRGB. They encode different brightnesses in the different color spaces so it is not at all a surprise that you see different results and a necessary result that has little to do with what Lightroom does.

It’s not just the gamut. A main difference for this purpose is in the color space’s gamma curve. prophotoRGB has a 1.8 gamma, while sRGB uses a complex gamma curve ( http://lagemaat.blogspot.com/2008/06/srgb-tone-curve-and-lightroom-color.html ) that is close to gamma 2.2 but not quite. AdobeRGB assumes a gamma curve with exponent 563/256, which is practically 2.2. adobeRGB does not have the linear part near zero which causes it to be very different in the dark regions from sRGB. This all means that 128 in any of the color channels encodes for a brightness that is 32% higher in prophotoRGB than in sRGB and adobeRGB so you can’t really compare equal numbers. What you have to do is afterwards in Photoshop, convert to another color space and compare there. if you are using 8-bit files, you might find some posterization in the pro photo files that is simply due to the much wider color space being used with only 256 levels.

I never use ProPhoto, but it is interesting to know that its native gamma is 1.8.Is there a relationship between this and the old Apple RGB?

Other than the gamma and both are RGB working spaces, none.

Maybe this pair of free video's will help, after you might consider ProPhoto RGB, especially part three of the first that talks about what a wider gamut working space provides in shadows of saturated color you can capture:

Part 1 discusses how the supplied Gamut Test File was created and shows two prints output to an Epson 3880 using ProPhoto RGB and sRGB, how the deficiencies of sRGB gamut affects final output quality. Part 1 discusses what to look for on your own prints in terms of better color output. It also covers Photoshop’s Assign Profile command and how wide gamut spaces mishandled produce dull or over saturated colors due to user error.

Part 2 goes into detail about how to print two versions of the properly converted Gamut Test File file in Photoshop using Photoshop’s Print command to correctly setup the test files for output. It covers the Convert to Profile command for preparing test files for output to a lab.

Part 3 goes into color theory and illustrates why a wide gamut space produces not only move vibrant and saturated color but detail and color separation compared to a small gamut working space like sRGB.

If you shoot in Raw, that camera setting is irrelevant because it applies only to jpgs made by the camera. Raw files are greyscale data files and therefore have no color and no color space. The color is created by Lightroom. At the time of outputting a rendered RGB image, either with Export or Edit In..., you choose the color space in which it will be rendered.

If, OTOH, you are editing jpgs in Lightroom, you could increase the saturation and thus create new (artificial) colors in the image that are beyond the gamut of the original jpg's space. Saving the new jpg in ProPhoto RGB would preserve those colors.

BTW, a smaller gamut does not mean "less info". It means the same amount of data refers to a narrower range of colors.

elie is right. The camera setting has no bearing on the final result from Lightroom if you shoot raw. It has only very limited effect if you shoot jpeg. You'll likely not be able to notice the difference in the jpeg case except perhaps if you have highly saturated colors in the image. Flowers are a good example. But again, the setting is not used at all when you shoot raw. In fact it is one of the most confusing settings in cameras that really for raw files should be disabled but I doubt camera makers will do that.

FYI, the way Lightroom works is that it regardless of the source file format, raw, jpeg, tif, psd, or png, and regardless of the source file's color space (if it is non-raw) renders the image into a 16-bit color space that has prophotoRGB primaries and a linear gamma. This ensures no posterization artifacts can occur. This means that for raw files, you always get the entire gamut the camera can capture and more if you consider that vibrance, saturation, and such can send the colors far further into lala land. These colors are rather unlikely to run out of the linear prophotoRGB space as that is incredibly wide gamut. It then converts the file from the linear prophotoRGB space to whatever color space it needs to output, be it sRGB, adobeRGB or 1.8 gamma prophotoRGB for export, or the display profile for displaying or a printer profile for sending to a printer. In all cases, you'll get the best representation from the native linear prophotoRGB space. This includes any noise present in the original, which will go through all these conversion steps and will come out the other end the same regardless of the output color space. It will just be as faithfully represented as the target space will allow.

Reading again, does it mean, that whatever be the input image, in LR working space is prophoto?

And since (assuming the answer is yes to above) it is prophoto, it is only when I output that it matters what I choose. Does this imply that AdobeRGB is fine and "output for printer profile" will use the best possible (LRs internal/working prophoto gamut)?

And since (assuming the answer is yes to above) it is prophoto, it is only when I output that it matters what I choose. Does this imply that AdobeRGB is fine and "output for printer profile" will use the best possible (LRs internal/working prophoto gamut)?

All processing in LR/ACR is done in a color space similar ProPhoto RGB (same gamut, but with a different 'gamma', Tone Curve). Unless you setup a soft proof, the histogram and numbers are based on ProPhoto RGB gamut (primaries).

Adobe RGB is fine for some output, too small for other output. See:

Everything you thought you wanted to know about color gamut

A pretty exhaustive 37 minute video examining the color gamut of RGB working spaces, images and output color spaces. All plotted in 2D and 3D to illustrate color gamut.